Devices for storing braking energy, and then drawing on the stored energy to accelerate, are known in the art. Most past efforts to provide regenerative braking were based on flywheels or metal springs. It has been suggested that because flywheels exhibit a far higher ratio of energy storage capacity to weight than do metal springs, flywheels would therefore be a preferred energy storage medium. See generally, Whitt et al., Bicycling Science, second edition, MIT Press, pp. 315-318 (1982). However, flywheel systems are heavy, require a complex continuously variable transmission to efficiently deliver energy, and have high energy dissipation due to windage and other losses.
Devices for storing energy, using a metal spring, are disclosed in U.S. Pat. No. 784,345 to Pepper, and U.S. Pat. No. 2,965,393 to Cauchon. The Pepper device has the problem of requiring a large wheel to be flipped 180 degrees for energy redelivery. Both prior art devices have the problem inherent in the use of a metal spring: a metal spring has a low energy storage capacity per unit weight, and is therefore unsuitable for use on a modern lightweight sport bicycle. Another problem of these prior art devices is their cumbersome size and location, characteristics which reduce their utility in applications affording limited space. An additional problem with these prior art devices is that their energy storing transmissions frictionally engage the tire of the vehicle's drive wheel, thereby introducing an element of lossage in braking and energy storage, caused by wear or slippage, especially under high load or adverse weather conditions. High spring forces would be necessitated in order to ensure engagement, further exacerbating wear problems. Yet another problem with the prior art devices is that no provision is made for maintaining a constant, predictable level of deceleration and acceleration force as the force of the springing medium increases and decreases during storage and delivery of energy. Further, a sudden and surprising increase in deceleration force experienced as the springing medium reaches the limit of its travel could pose a hazard to the rider, especially in braking during high speed descents or upon entering decreasing radius turns at speed.